It is expected that high temperature superconducting cables will be used for supplying a large volume of electricity. Particularly in urban areas having a restricted space, it is difficult to ensure routes (which are referred to as “duct lines”) for laying new cables. Even if this is physically possible, enormous infrastructural cost in terms of construction would be required, which makes such cable laying infeasible.
In order to solve this problem, high temperature superconducting cables having a small size almost equal to the size of known crosslinked polyethylene (XLPE) cables and which are capable of supplying the amount of current several times greater than the known XLPE cables are being developed. In a prototype of cable system made in the past, a three-phase core high temperature superconducting cable having a multiple layers of Bi based superconducting silver sheathed tape wires wound around a former, an insulating layer and a shielding layer is inserted into a long piece of thermal insulation tube, into which a cooling medium, such as liquid nitrogen, is supplied.
Measures against a short-circuit current caused by thunderstorms or disasters have been taken for XLPE cables designed to be placed underground. In practice, such measures against a short-circuit current are also essential for high temperature superconducting cables. Thus far, the following techniques have been considered: {circle around (1)} a technique for causing a short-circuit current to flow into a copper former; and {circle around (2)} a technique for making the flowing current capacity of the cable cores much greater than that during normal operations so that high temperature superconducting cable cores are prevented from being damaged in the occurrence of a shirt-circuit current. Additionally, {circle around (3)} a technique for inhibiting a short-circuit current and also for protecting a high temperature superconducting cable by connecting a superconducting apparatus called “current limiter” to the cable is also considered.
However, the technique for shunting a short-circuit current with the former results in an increased cross sectional area of the former, and thus, a small size, which is unique to the high temperature superconducting cables, is impaired.
The technique for making the flowing current capacity of a high temperature superconducting cable core much greater than that during normal operations, results in increased cost of the cables.
The development of current limiter is at the fledgling stage of consideration, and there are various problems to be solved in terms of the system and materials for the development of its high voltage system that can be applied to a cable system of 66 kV, or higher 275 kV.